Some common German verbs have a vowel change in their stem when you use them in the present tense. This change only happens for the second person singular 'du', and the third person singular 'er', 'sie', and 'es'. The good news is that the verb endings are the same regular ones you already know, and all other forms conjugate normally.

Let's start with the verb 'essen', which means 'to eat'. For 'du' and 'er, sie, es', the 'E' in the stem changes to an 'I'. So, while 'I eat' is 'ich esse', 'you eat' becomes 'du isst' and 'he eats' is 'er isst'. The other forms like 'wir essen' and 'ihr esst' are regular.